1. Technical Field
The present invention relates to a hydrogen storage alloy electrode which can electrochemically absorb therein (charge) and desorb therefrom (discharge) hydrogen and is applicable to storage batteries such as nickel-metal hydride storage battery.
2. Background Art
Recently, nickel-metal hydride batteries including a hydrogen storage alloy electrode as the negative electrode have rapidly become popular in our daily life because of their use as the secondary battery of compact portable appliances such as personal computer and cellular phone. The nickel-metal hydride battery is characterized in that it has a 1.5- to 2-fold higher capacity and is more pollution-free than the conventionally and widely used nickel-cadmium storage battery because of no use of poisonous cadmium. More recently, in addition to compact portable appliances, application and inclusion of the nickel-metal hydride storage battery has been widened even to electric vehicle (EV) and hybrid electric vehicle (HEV) and further to electric instruments and emergency lights to which the nickel-cadmium storage battery has conventionally been applied widely.
A conventional type of hydrogen storage alloy electrode used in the nickel-metal hydride storage battery has been produced mainly by a paste coating technique. Such paste electrode is obtained by a method which mixes a hydrogen storage alloy powder, organic binders such as styrene-butadiene rubber (SBR) and/or carboxymethyl cellulose (CMC) and a conductive powder such as carbon, kneads the resultant mixture into a paste, then coats the paste on both surfaces of a low cost conductive core member made of punched metal such as nickel-plated iron sheet and subsequently dries, presses and cuts the conductive core member to make an electrode (see the publication of the U.S. Pat. No. 5,527,638, for example).
The paste electrode produced by the paste coating technique is relatively specific to mass production and there are many attempts to realize the mass production. However, the paste electrode still has drawbacks of unsatisfactory high rate charge/discharge performance and unfitness for use in making instantaneous charge and discharge at a large current.
Another type of electrode is an electrode obtained by a method called sintering. The Japanese-Laid Open Patent Publication No. Hei 3-294405, for example, discloses a method for continuous manufacturing of negative electrodes which supplies a hydrogen storage alloy fine powder to a wire mesh screen, then compresses the screen to form a deposit and sinters the deposit, followed by quenching the sintered deposit in a gaseous hydrogen atmosphere. In this method, although improved high rate charge/discharge performance can be expected as compared with the paste type electrode, it also has drawbacks of much laborious process and liable reduction of alloy performance due to sintering, which have disturbed wide use of sintered electrode.
Apart from the paste coating and sintering methods, a method has been proposed in, for example, Japanese Laid-Open Patent Publication No. Sho 62-216163 which obtains an electrode by molding a mixture of a hydrogen storage alloy with a fluorocarbon resin binder into a sheet and then mechanically pressing the sheet against a current collector. However, the electrode obtained by this method has a problem that it catches fire easily, in addition to the conventional problem of unsatisfactory high rate charge/discharge performance.
The electrode obtained by any of the above-mentioned methods described in the publication of the U.S. Pat. No. 5,527,638 and Japanese Laid-Open Patent Publication No. Sho 62-216163 includes some form of organic binder which is used as an essential constituent in the process of electrode production. The organic binder can be a factor for increasing electrode resistance.
To the contrary, as the method which does not use the organic binder as the factor for increasing electrode resistance, a method of producing electrode by a dry press technique using a scaly copper or nickel powder has been suggested in two Japanese Laid-Open Patent Publications No. Hei 7-307154 and Hei 9-245797, for example. However, the electrode obtained by this method is also disadvantageous in that due to insufficient contact between the hydrogen storage alloy portion and conductive metal portion, an absolute content of the conductive material must be increased in order to improve high rate charge/discharge performance. This type of electrode is considered to have another problem in terms of capacity density per electrode volume.
Despite many other proposed hydrogen storage alloy electrodes and methods for producing the same, there has been an increasing demand for a simple and low cost method which can readily offer an electrode with particularly excellent output characteristics.
Generally required conditions for the hydrogen storage alloy electrode for use in the nickel-metal hydride battery and so on from the aspect of the intended use include: (i) excellent high rate charge/discharge performance; (ii) high durability and long service life; (iii) high energy density; (iv) high conductivity; (v) high mechanical strength and ruggedness; (vi) simple, low cost and easy production method of high utility; and (vii) easy-to-recycle.
It is true that hydrogen storage alloy electrodes obtained by the previously described prior art methods may satisfy the above conditions considerably; however, there still exists a demand to further improve the conditions (i), (vi) and (vii) in response to recent market requests in particular.
Therefore, the primary object of the present invention is to provide a novel hydrogen storage alloy electrode which has excellent high rate charge/discharge characteristics and facilitates recycling by using a low cost method of high utility.
The present invention is characterized by a hydrogen storage alloy electrode comprising a hydrogen storage alloy and a conductive metal and completely free of organic binder, which is formed by integrating at least two layers of an active material holding layer essentially composed of a hydrogen storage alloy powder and a conductive metal powder and a conductive metal layer essentially composed of the conductive metal into a sheet and is imparted with a conductive network communicating throughout the electrode.
In the context of the present invention, the intended meaning of the xe2x80x9ccomprisingxe2x80x9d or xe2x80x9cessentially composed ofxe2x80x9d is that other components may also be contained to an extent not to injure the effect of the present invention.
In a preferred mode of the present invention, the conductive metal comprises Ni or Cu or an alloy containing Ni and Cu, and the active material holding layer essentially composed of the hydrogen storage alloy powder and conductive metal powder comprises 70 to 95 wt % hydrogen storage alloy powders and 30 to 5 wt % conductive metal powders and the conductive metal layer comprises 95 wt % or more conductive metal.
In another preferred mode of the present invention, a center of the electrode is essentially composed of the conductive metal layer and both ends of the electrode are essentially composed of the active material holding layer in a direction of electrode thickness.
Inversely, the center may be essentially composed of the active material holding layer and both ends may be essentially composed of a porous conductive metal layer in a direction of electrode thickness.
In another preferred mode of the present invention, compositions of the conductive metal layer and the active material holding layer are inclined in continuity in a direction of electrode thickness.
Further, both ends of the electrode are preferably plated and have conductive metal layers.
In a further preferred mode of the present invention, at leaset one portion of the conductive metal layer is composed of a two-dimensional or three-dimensional porous metal. The two-dimensional or three-dimensional porous metal is desirably an embossed plate, punched metal sheet, expanded metal sheet, foamed metal sheet, lath metal sheet or metal fiber cloth.
Alternatively, it is also preferable that at leaset one portion of the conductive metal layer is composed of a metal foil.
One end or both ends of the electrode may be composed only of the conductive metal layer in a direction of electrode width.
In still another preferred mode of the present invention, the electrode has a thickness of 0.5 mm or less and a porosity of 5 to 20%.
It is also preferable that the hydrogen storage alloy powder has a nickel-rich surface by pretreated with hot alkali or acid.
It is also desirable that the hydrogen storage alloy powder is subjected to mechanofusion or plating beforehand and has a surface having a metallic nickel layer.
The present invention also relates to a method for producing hydrogen storage alloy electrode comprising a hydrogen storage alloy and a conductive metal and completely free of organic binder, comprising the steps of:
(a) supplying a hydrogen storage alloy powder, a conductive metal powder and/or a porous conductive metal;
(b) laminating at least two layers of an active material holding layer comprising a mixture of the hydrogen storage alloy powder and/or conductive metal powder and a conductive metal layer essentially composed of the conductive metal powder or porous conductive metal; and
(c) pressing a laminate produced by the previous step (b) to integrate the active material holding layer and conductive metal layer into a sheet and to produce a conductive network communicating throughout the electrode.
In the above-mentioned production method, step (b) and step (c) are preferably performed concurrently.
It is particularly desirable that the step (b) and step (c) are performed concurrently with a roll-press method using a pair of rolls whose surfaces have uneven parts.
It is also preferable that the mixture is heated in a non-oxidative atmosphere for 10 minutes or less in a temperature range of not less than 500 C and not more than the lowest melting point of the melting points of the metal elements constituting the electrode during or after step (c).
In that case, a desirable heating method is induction heating, electrical resistance heating, hot-press heating, or light beam heating or heat ray irradiation heating.
The present invention also provides a battery including a hydrogen storage alloy electrode comprising a hydrogen storage alloy and a conductive metal and completely free of organic binder, characterized in that the hydrogen strange alloy electrode is formed by integrating at least two layers of an active material holding layer essentially composed of a hydrogen storage alloy powder and a conductive metal powder and a conductive metal layer essentially composed of a conductive metal into a sheet and has a conductive network communicating throughout the electrode.